#include <iostream>

using namespace std;

/**
 * https://www.luogu.com.cn/problem/P2669
 */
int main(){

    int n;
    cin >> n;

    int Sn = 0;
    int cnt = 0;
    int i = 1;
    while(true){
       int j=i;
       while(j--){
          Sn += i;
          cnt++;
          if (cnt == n){
            cout << Sn;
            return 0;
          }
       } 
       i++;
    }
    return 0;
}